You probably get a lot of cool ideas, thoughts that juice you up and feel Just Right for you and your circumstance. I am willing to bet you typically write down the idea, talk about it with friends, think about it for a few days, then.... nothing. You forget about it or it loses its lustre. You move on. You probably have a mental shelf full of never-wases and could-have-beens, ideas that sparkled for a short while but never made it.
The idea is a seed. You need to plant it and germinate it before it's too late. If you don't - if you leave the seed on the shelf - it will start to lose its life and take rot. An idea not acted upon will seem dull two weeks later.
Whenever you feel juiced up by a brilliant new idea, immediately open a text file and dump everything you can think.of pertaining to that idea. Pack that initial inspiration into solid ground.
Paradoxically, this will rob you of that initial juice. You will feel "meh" about the idea for a little while. BUT... and this is big, you now have germinated the seed and over the coming weeks you will notice it growing. You will receive more and more inspiration and follow-on ideas and your text file will fill up. People will start suggesting things you could add.
This is when you go public with your idea and start saying self-concept building things like "I am currently writing a book / programming an app / starting a blog / etc". Because it will be true then, and it will fuel you.
And soon enough, your seed will be a stout oak.
(Btw, I followed my own advice with this post. Written and posted in 15 minutes upon having the idea.)